In the frame of the 3GPP HSPA evolution work, there are initiatives to extend the use of the HS-DSCH (High-Speed Downlink Shared Channel) downlink shared transport channel. Improvements are being made to the Cell_FACH and UTRAN (UMTS Terrestrial Radio Access Network) paging states. Targets are extensive use of the HS-DSCH transport channel in the aforementioned states.
There are several incentives for extending the use of the HS-DSCH channel. In particular, it is note that the current common channels (PCH, FACH and BCH) are mapped on physical channels that consume both codes and power. If (parts of) this common channel traffic can be mapped onto HS-DSCH, then static code and power reservation can be avoided and a more flexible system can b achieved. This would particularly be true in, for example a 5 MHz band where only “evolved HSPA” UEs (User Equipments) would be served, i.e. in a situation where the operator keeps “legacy” UEs (that need a secondary CCPCH) in a different band.
An equally important benefit of the HS-DSCH channel is its flexibility. Compared to current FACH, much higher bit-rates can be achieved over HS-DSCH, which would facilitate shorter channel setup-times. Long delays in channel setup (state changes) are one of the main performance bottlenecks in today's WCDMA system.
However, a problem with the present FACH realization is that it lacks means for link adaptation. In other words: since the FACH must reach all UEs in the cell, the modulation, coding and power has to be set in a conservative manner so that the FACH channel can be heard at the cell edge. This means that the FACH channel consumes a lot of resources, since it needs to be designed with the worst-case in mind. Observe that the FACH channel is currently the point-to-multipoint channel used for MBMS (Multimedia Broadcast and Multicast Service).
The HS-DSCH has support for advanced link adaptation. However, this is true only in Cell_DCH state, because the link adaptation relies on (frequent) link quality reporting (CQI—Channel Quality Indicator) in the uplink: The UE estimates the downlink quality based on DL pilots and send CQI reports to the Node B. Based on the CQI, the Node B can then select the most appropriate resource allocation, where this resource allocation can be chosen in the dimensions of:    1. modulation & coding (reflected in the selected Transport Format)    2. power (i.e. how much of the power resource is used for this transmission)    3. number of codes (number of HS-PDSCH physical channels) used for the transmission.
The CQIs are sent over the UL HS-DPCCH, which is available in Cell_DCH only.
Thus, the Node B has several degrees of freedom in how to respond and act to a certain CQI reported by an UE. For example, for a fixed CQI (link quality) the Node B could either
a.) use less codes, less power, but strong modulation and coding, or
b.) use more codes, more power but weaker modulation and coding.
This choice could depend on the resource situation in the Node B—depending e.g. on whether the cell is code or power-limited.
In the UTRAN paging and FACH states there are, however, very limited means to provide any uplink (CQI) feedback to the network. The HS-DPCCH is not there. Due to the costs of maintaining links for dedicated CQI reporting in the UL, it does not appear feasible to implement such elaborate link adaptation in FACH/paging states. The cost of maintaining an uplink for transmitting reports is simply too high considering that the UEs are supposed to be quite idle in Cell_FACH/paging states.
Although, in the description above, a very specific technology implementation, i.e. the ongoing work with the Cell_FACH state in 3GPP, is discussed, the problem and the solution, however, are easily extended to any access technology. In particular and for example the LTE work where there is an equal need to have adequate forward-access link-adaptation particularly for MBMS, but possibly also for System Broadcast Information, Paging etc.
It is known, e.g. from Multimedia Broadcast and Multicast Service (MBMS) work, that the resource allocation (e.g. Modulation and Coding Scheme (MCS) and power) should be chosen according to the UE with the worst link budget. Existing solutions for such link adaptation of a channel received by multiple users all rely on regular feedback from the receivers (UEs). This means that the UE sends a signal or negative acknowledgement when it failed to receive the multi/broadcasted channel, so that the transmitter can adjust its resource allocation. Note, however, that    a.) the sender then has to “probe” the worst user resulting in reduced performance for the suffering receiver.    b.) the current UTRAN specifications do not support any link adaptation for MBMS carried over FACH.
Thus, current MBMS suffers from the problem depicted above. It is not desired to waste resources on frequent feedback on the reverse link, but only information on the link quality of the worst user is needed. It is also desired to also provide the worst user decent received quality without over-allocating resources.
JP 2003188818 relates to a mobile communication system which includes a base station and many user equipments capable of communicating with the base station in a cell occupied by the base station and where the base station can broadcast multicast multimedia broadcast service data to a plurality of pieces of the user equipment among many of the user equipment. The base station receives channel quality information of each user equipment from a plurality pieces of the user equipment and increases or decreases the transmission power of the base station to control the transmission power of the base station on the basis of the worst channel quality information among the channel quality information items received from a plurality pieces of the user equipment in order that the base station controls the transmission power to a plurality of the user terminals for broadcasting data. However, this solution assumes that each terminal is sending their “CQI”.
Yet another problem occurs, for example in MBMS, which is a new service. The aim is to transfer data such as audio, video and interactive data simultaneously to several users in the downlink. The MBMS data is transmitted over a common transport channel such as the Forward Access Channel (FACH), which in turn is sent over the Secondary Common Control Physical Channel (S-CCPCH). The aim is also to use unacknowledged mode link layer, thereby relying only on Forward Error Correction (FEC).
The common channels do not have any closed loop power control mechanism. In fact, the FACH power is tuned with respect to the Common Pilot Channel (CPICH) power. These power-offset levels are usually obtained using static radio planning tools. Therefore, currently fixed offset is used between FACH and CPICH power levels. Even if the CPICH power level is tuned dynamically, the power-offset between FACH and CPICH remains fixed from the onset of the installation.
The power should be set according to the distribution of the MBMS users in the cell. Ideally the transmit power level should be optimum in a way to provide adequate quality to all the broadcast/multicast recipients, while at the same time prevents unnecessary interference to other users and to other cells.
The current solution of fixed power setting is designed for the worst possible scenario. The existing method does not take into account the dynamics in the system such as varying load and coverage.
Drawbacks include:                Too high power setting leads to unnecessary interference to the own cell users as well as to users in the neighboring cells.        Similarly very low power level may prevent some users from correctly receiving the broadcast data due to higher block error rate at the user equipment (UE) receiver. Low power level may even cause demodulation problems at the receiver.        
Since MBMS utilizes unacknowledged mode link layer therefore optimum power setting method providing adequate quality is of great significance.